A polymorphic value-type for C++

The class template polymorphic_value is proposed for addition to the C++ Standard Library.

The class template, polymorphic_value, confers value-like semantics on a free-store allocated object. A polymorphic_value<T> may hold an object of a class publicly derived from T, and copying the polymorphic_value will copy the object of the derived type.

Using polymorphic_value a copyable composite object with polymorphic components can be written as:

// Copyable composite with mutable polymorphic components
class CompositeObject {
  std::polymorphic_value<IComponent1> c1_;
  std::polymorphic_value<IComponent2> c2_;

 public:
  CompositeObject(std::polymorphic_value<IComponent1> c1,
                  std::polymorphic_value<IComponent2> c2) :
                    c1_(std::move(c1)), c2_(std::move(c2)) {}

  // `polymorphic_value` propagates constness so const methods call 
  // corresponding const methods of components
  void foo() const { c1_->foo(); }
  void bar() const { c2_->bar(); }

  void foo() { c1_->foo(); }
  void bar() { c2_->bar(); }
};

Submodules

Tests use the 'catch' test framework: https://github.com/philsquared/Catch.git

To get the submodule run:

git submodule update --init

Building

The build uses cmake driven by a simple Python script. To build and run tests, run the following from the console:

./scripts/build.py --tests

Build status

• on Travis:
• on AppVeyor:

ISO Standardisation

polymorphic_value has been proposed for standardisation for C++20 in P0201: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0201r1.pdf. The draft in this repository is more up to date than the paper linked above, in particular the class has been renamed from indirect to polymorphic_value.